DICTIONARY OF

Asteroids 24 Astronomical Twilight
Asteroids. Large number of very small planetary bodies, the mean point of whose orbits lies
between Mars and Jupiter at approximate distance (2-8 astronomical units) required by
Bode's Law. Orbits of about 1200 have been observed; between 200 and 300 have been
named; many have been observed for short periods only. Vesta is the only one discernible by
the naked eye. The largest, Ceres, has a diameter of 480 miles; the majority are very much
smaller.
Astigmatiser. Lens and mechanism, in a rangefinder, that causes a point of light to appear as
a line of light; which can then be used to determine distance off.
Astres Fictifs. Name given to hypothetical bodies/assumed to be responsible for component
tides in harmonic analysis. More usually termed 'satellites' or 'constituents'. Fr. = 'False
stars'.
Astrolabe. Probably oldest of navigational instruments. In its more advanced form consisted
of a disc of metal, or wood, graduated in degrees and suspended from a ring representing the
zenith. A movable bar, or alidade, pivoted at centre of dial, carried sight vanes that were
aligned on observed body. Bar then indicated altitude. In all cases, vertical and horizontal
planes were established by plumb line. 2. Optical instrument formerly used in surveying.
Now superseded by theodolite.
Astro-Meteorology. Investigations into effects of Sun and Moon on weather.
Astro-Navigation. Conducting of a ship by observations of heavenly bodies, as
distinguished from observations of terrestrial objects.
Astronomical Bearing. True bearing of a terrestrial object when derived from angle
between a vertical circle passing through the object and another vertical circle passing
through a heavenly body whose azimuth has been computed.
Astronomical Clock. Timepiece regulated to measure sidereal time.
Astronomical Cross Bearings. Crossed position lines obtained from observations of
celestial bodies.
Astronomical Day. Interval between successive transits of mean Sun across a given
meridian. Since 1924 it has coincided with the civil day: before then it commenced with
Sun's superior transit and was, therefore, 12 hours slow on civil time.
Astronomical Position Line. Line of position obtained by observation of a heavenly body.
Astronomical Refraction. Difference between original direction of a ray of light, from a
heavenly body, and its final direction when it reaches observer. Due to refractive effect of
atmosphere.
Astronomical Tide. Alternative name for 'Equilibrium Tide'.
Astronomical Time. Time measured directly by hour angle of mean Sun: noon was thus 00
hours. Previous to January, 1925, this time was basic in navigation.
Astronomical Twilight. Interval between Sun's centre being 12° below horizon and 18°
below it. Horizon is not distinct but vestiges of sunlight are refracted or reflected above it.